1. Field of the Invention
The present invention relates to a semiconductor device fabricated by using a solder capable of maintaining a reliable bonding ability at high temperatures and an automotive ac generator (alternator) provided with the semiconductor device.
2. Description of the Related Art
As mentioned in JP-A 07-221235 (Patent document 1) by way of example, a semiconductor device for an automotive ac generator is constructed so as to reduce thermal stress that is induced therein due to difference in thermal expansion between the semiconductor device and electrodes so that the semiconductor device can withstand a severe thermal cycle. Since the automotive ac generator is installed near an engine, the semiconductor device included in the automotive ac generator is required to withstand a high temperature of 200° C. Therefore, the electrodes of the semiconductor device are soldered to circuit terminals with, for example, a high-Pb solder having a solidus around 300° C., such as a Pb—Sn alloy containing 95% by weight Pb and 5% by weight Sn and having a solidus of 300° C. and a liquidus of 314° C.
From a viewpoint of environmental protection, there is a demand for semiconductor devices using a bonding material not containing Pb which imparts a heavy load to the environment. An Au-20Sn solder (eutectic, 280° C.), an Au-12Ge solder (eutectic, 356° C.) and an Au-15S solder (eutectic, 363° C.) are Pb-free solders not containing Pb and having properties similar to those of high-Pb solders. However, those Au solders, namely, Pb-free solders, are very expensive. The Au-20Sn solder having a comparatively low Au content is a hard solder incapable of satisfactorily relaxing stress induced in a wire area and hence a semiconductor device having electrodes bonded to terminals by this hard solder is likely to break.
A Sn solder, such as a Sn-3Ag-0.5Cu solder having a melting point not lower than 200° C., is another Pb-free solder having a medium melting point. This Sn solder is used prevalently for mounting parts to a wiring board and has satisfactory bond reliability at temperatures not higher than 150° C. However, if parts soldered by this Sn solder are kept for a long time in a working environment of 200° C. or above, an interfacial reaction occurs in the interface between the bonded parts. Consequently, voids are formed and intermetallic compounds grow and the bond reliability is reduced.
A method of suppressing the interfacial reaction of the Sn solder disclosed in Jpn. Pat. No. 3152945 (Patent document 2) uses a Sn solder containing 0.1 to 2% by weight Cu, 0.002 to 1% by weight Ni and Sn as the remainder. It is mentioned in Patent document 2 that Cu contained in this Sn solder controls the Cu erosion of the materials of bonded parts and Ni contained in this Sn solder controls the growth of intermetallic compounds, such as Cu6Sn5 and Cu3Sn in interface between bonded parts. A method of forming a solder bump mentioned in JP-A 2002-280417 (Patent document 3) forms two kinds of metal layers that interact with a Sn solder and form intermetallic compounds on the surface of a part to be bonded to another part and bonds a Sn solder ball to the surface. It is mentioned in Patent document 3 that an interfacial reaction can be suppressed by thus forming a thin layer of an intermetallic compound containing two or three elements including Sn in the interface between the bonded parts.